1. Field of the Invention
This invention relates to a packaging material for photosensitive materials for photographic purposes.
2. Description of Prior Art
Various types of packaging materials for photographic photosensitive materials have widely been put to practical use, and various properties are required according to their uses.
As the packaging materials for photographic photosensitive materials, it is necessary that various properties be present such as light-shielding, slitability, gas barrier, moisture proofing, antistatic property, rigidity, physical properties such as breaking strength, tear strength, impact puncture strength, Gelbo test strength and wear resistance, heat sealing properties such as heat seal strength, cut sealability, hot tack properties (hot-sealability) and seal ability of contraries, flatness antiblocking, slipping character and the like. The slitability is the property capable of cutting film smoothly without jag or rupture. The cut sealability is the suitability for fuging seal. In this sealing method, the hot plate for press is changed to a knife-shaped, and melting adhesion and cutting occur simultaneously.
Generally, it is difficult to satisfy these properties by a single material. Therefore, a single layer film of a high-pressure low-density polyethylene (LDPE) kneaded with carbon black or a pigment, and a composite laminated film composed of a LDPE film and a flexible sheet such as paper, aluminum foil or cellophane, etc. have been employed. An example of the conventional laminated film is shown in FIG. 8. This film is composed of a light-shielding LDPE film layer 6a, a light-shielding metal foil layer 7a laminated on it through an adhesive layer 3, and a flexible sheet layer 4 laminated thereupon through an adhesive layer 3. Another example of the conventional film is shown in FIG. 9. This film was used for packaging color photographic printing paper, and it is composed of a light-shielding LDPE film layer 6a, a light-shielding metal foil layer 7a, a flexible sheet layer 4, and a light-shielding LDPE film layer 6a. They are laminated in that order, and an adhesive layer 3 is provided between each layers.
On the other hand, the present inventor has already disclosed a laminated film for photographic photosensitive materials of which physical strength was improved by combining two uniaxially stretched films (U.S. Pat. No. 4,331,725). With respect to the packaging material containing a low-pressure linear low-density polyethylene (L-LDPE), a packaging material comprising at least one light-shielding film layer of polyethylene polymer of which more than 50 wt. % is L-LDPE and containing more than 1 wt. % of a light-shielding material has been disclosed (U.S. Pat. No. 4,452,846).
Physical properties of the conventional laminated films are not enough, and during packaging, the films were sometimes torn or perforated or heat sealing of the films was sometimes separated. In addition, when a large amount of a light-shielding material such as carbon black is added, physical strength of the film is lowered. Then, the amount was set about 3%, and thickness of the film was more than 70 .mu.m. As a result, the film was stiff, and working efficiency of the packaging process was badly influenced. Cost of the packaging was also expensive.
In the case of the laminated film dislosed in the specification of U.S. Pat. No. 4,331,725, heat sealing properties are not good because of using uniaxially stretched high-density polyethylene film, and problems in sealing occasionally happened in packaging processes. In the case of cross-laminated films where two uniaxially stretched films were directly laminated so as to cross each other without laminating an aluminum foil, when a photosensitive material was packaged in a gusset bag, pinholes were appreciably formed at its gusset portion.
By employing L-LDPE resin for the packaging material, physical strength such as tear strength and Gelbo test strength are raised and blending amount of a light-shielding material can be increased. However, when thickness of the film was thinned, crumples and creases occurred, and tensile strengh, slipping character, rigidity, moisture proofing, gas barrier, production aptitude, etc. became insufficient. On the other hand, its slitability and cut sealability were inferior.
Particularly, as the packaging materials for X-ray films for dental use, as the packaging materials for radiograph films for detecting radiation exposure of a person who treats radioactive materials (badge film) and as the packaging materials for vacuum packaging of the X-ray films described in Japanese Patent KOKOKU No. 55-24089, the development of single films capable of satisfying all properties required as the above packaging materials has been desired.